I have a particular opinion when it comes to closing a question as a duplicate specifically, which is a bit different than the other close reasons (e.g. a duplicate should not be put on the path of deletion).
Marking a question as duplicate is a matter of improving navigation for everyone on the site. A duplicate is not necessarily a bad thing, because the same question can be asked in many different ways and often it is difficult to find a question with a specific wording if you used different search terms. If after a search you land on a question which is marked as a duplicate, it will be much easier to go to the other question that contains helpful answers as well as links to every other duplicate, which might contain even more helpful answers. Closing as duplicate is also not "punishment" to the OP in any way (a priori). So, the age of the question and the fact that the OP is no longer active are completely irrelevant.
I agree with the comment that "sufficiently old questions don't need to be held to the standards of today", but I disagree that this implies that old questions shouldn't be marked as duplicates if they are.
Again, this is because I see closing as duplicate not as judging whether a question is "high quality" or "bad quality", but rather it is purely a matter of improving navigation on the site, i.e. improving the site for everyone.
(Other close reasons might be a different story though.)